In a manufacturing process of a semiconductor device, when performing thermal oxidation processing or CVD deposition processing on an Si wafer to be a product, dummy wafers or monitor wafers are usually mounted on a wafer boat together with the product Si wafers. The dummy wafers are added when the product wafers are still insufficient in a specified number of the wafers, or inserted to make a temperature or a gas flow inside a furnace uniform in the CVD process and the like, and therefore, the dummy wafer can not be a final product. The monitor wafer is processed by completely the same processes as those for the product Si wafer, to monitor a film thickness of a CVD film or the like which is formed on the product Si wafer, and the number of particles in the furnace. Accordingly, especially the product Si wafers and the monitor wafer are placed on a wafer boat together, and thermal oxidation films or CVD films are formed simultaneously on the surfaces of the product wafers and the monitor wafer through predetermined processing.
After completion of the predetermined processing, the film thickness measuring monitor wafer is taken out from the boat. Then, a film thickness of the film formed on the monitor wafer is measured with a film thickness measuring instrument to judge whether or not the measured film thickness is within a spec. If it is within the spec, it can be judged that a film having a film thickness within the spec is formed on the product wafer processed at the same time, so that the product wafer can be transferred to a next process.
Incidentally, as a film thickness measuring monitor wafer described above, conventionally, an Si wafer made of the same material as the product wafer has been usually used. When this Si wafer is used as a film thickness measuring monitor wafer, the film is removed by polishing or etching, or a combination of polishing and etching every time when the film thickness measurement is finished, so as to render it reusable. For instance, in a D-poly deposition process for a six-inch wafer, a D-poly film having a thickness of 1000 Å is formed on the film thickness measuring monitor wafer in one time. Once formed, the D-poly film is removed with HF+HNO3+H2O (1:8:12) to use it as a film thickness measuring monitor wafer again. Since the surface becomes rough after repeating this process three or four times, it cannot be used as a film thickness monitor wafer. Besides, after repolishing three or four times repeatedly, since the thickness of the wafer becomes thin due to the polishing, it is easy to be deformed or cracked. Or, it becomes unmeasurable, because of getting rougher of Si surface, which makes the wafer unusable. Accordingly, such a wafer is used as a dummy wafer, not as a monitor wafer, or is scrapped.
Therefore, when the above-described Si wafer is used as a film thickness measuring monitor wafer, it has a problem that it cannot be used anymore as a monitor wafer after several times of usage, which results in a short life as a monitor wafer. From such a point of view, recently, a film thickness measuring monitor wafer made of silicon carbide (SiC) has been noticed by fields concerned. More concretely, the SiC monitor wafer is a disciform silicon carbide film having a film thickness of about 1 mm formed by a CVD method, has many following advantages written below and practical use of the SiC monitor wafer is promoted due to its prominent economical effect.
a) It is excellent in corrosion resistance against nitric acid, hydrofluoric acid, and the like, which makes it easy to remove adherents by etching, and the surface of the monitor wafer itself is not roughened by etching, which makes it possible to use repeatedly for a long time.
b) Since difference in thermal expansion coefficient between SiC and silicon nitride or between SiC and polysilicon is small, a film adhered on a monitor wafer is not easy to be peeled off, which makes it possible to reduce particles remarkably on the way of process.
c) Since it is extremely low in diffusion coefficient of impurities such as heavy metals at high temperatures, there is little apprehension of furnace contamination.
d) Since it is excellent in thermal deformation resistance, it facilitates automatic transfer such as robot transfer.
However, when the above-described SiC wafer is used as a monitor wafer, the same surface roughness as that of a product Si wafer is fundermentally required. Especially for measuring particle, though the same surface roughness as that of an Si wafer is required, it is very difficult to polish the SiC wafer to the same degree as the Si wafer. The polishing is performed by chemical mechanical polishing (CMP) when it is required. At this time, advantageous points of high mechanical strength and strong chemical resistance of the SiC wafer serves reversely as an obstacle to polishing. Therefore, there arises a problem in making it practically usable for using a SiC wafer as a monitor wafer so as to be able to measure particles and a film thickness at the same time.
However, there are two cases of usage for a monitor wafer. One is to measure particles and a film thickness simultaneously, and the other is to measure a film thickness only. The latter also appears in many cases.
When specifications required for a film thickness monitor are considered, it is found that the basic principle of the film thickness measuring method using a film thickness monitor wafer is an application of an interference color measuring method which utilizes a phenomenon that an interference color changes in accordance with change in refractive index or thickness of the film. From this point of views, the inventors reach the conclusion that the film thickness measurement is possible so far as it is a surface to obtain necessary reflection intensity (reflection coefficient) required for the measurement to incident light, and therefore a super mirror-finish surface (Ra=0.0002 μm) which corresponds to Si wafer level smoothness is not necessary.
Needless to say, not only SiC, alumina, quartz, tungsten, molybdenum, and further glassy carbon and sapphire can be cited as a raw material for a monitor wafer for measuring film thickness and the above basic principle can be applied to these materials.
The object of the present invention is to provide a wafer having a structure of enabling a hard wafer such as an SiC material to be put to practical use as a wafer for monitoring a film thickness.